The thing is, no other profession to my knowledge has such a system.
An analogy:
Take Terry Leahy (Tesco's CEO), very successfull businessman, worked his way up from the shop floor (literally).
Now, for the sake of argument, let's say he decided to go and work for Sainsbury's - he's hardly likely to join stacking shelves and working the checkout is he?
So, let's say I'm a hoary old skipper with 15,000 hours, having worked with company X for 20 years.
For whatever reason, I may have to change company - is it fair and reasonable that when I join a new company, I am junior to some Second Officer fresh out of training school with 200 hours?
Surely the greatest leveller in aviation is EXPERIENCE. We have a natural seniority inbuilt - hours flown. Isn't this fairer than an arbitary date of joining?